This invention relates generally to feedback control for turbine driven high powered alternating current synchronous generator systems. This invention relates specifically to generator systems of the above type which are utilized to provide electrical power for loads over relatively long transmission lines where the effect of series connected compensating capacitors may introduce torsional oscillation into the mechanical portion of the power generating system. It has been found that in long line electrical transmission systems of the type where the electrical generators are driven from power sources such as steam turbines, multiple rotating masses may be present on a single shaft. As an example the rotating mass of one or more turbines and the rotating mass of an exciter for an AC generator as well as the rotating mass of the generator rotor may be present. It has also been found that because of the mechanical properties of the various parts of the rotating system, torsional oscillation may be possible between any combination of masses. If these torsional oscillations were to become large and undamped they could cause shaft breakage. It has been found in certain applications that such undamped torsional oscillations may exist because of the electrical properties of the AC synchronous generator electrical system. As an example when long transmission lines are required for the delivery of energy to a grid network or load, the inductive effect of the long transmission lines may require series capacitive compensation. However, the effect of the compensating capacitors being connected in series circuit relationship with the inductance of the long transmission lines tends to cause certain resonant frequencies to exist in the output electrical circuit of the AC generator. These resonant frequencies may be reflected by the magnetic coupling between the stator and rotor of the synchronous generator to the mechanically rotating shaft. If the previously described electrical resonant frequency results in a critical mechanical resonant frequency between any of the masses of the generating system, reinforced undamped oscillation may occur which may be of sufficient magnitude to break the shaft. Apparatus and method for solving this problem have been proposed in U.S. Pat. No. 3,662,251 issued May 9, 1972, to O. J. M. Smith entitled "Method and System for Measuring Acceleration and Velocity". This latter patent is related to the sensing of oscillations rather than providing a method for directly correcting for the effect of the oscillation. A similar measuring method and apparatus is described in U.S. Pat. No. 3,662,252 issued May 9, 1972, to O. J. M. Smith and entitled "Tachometer and Method of Obtaining a Signal Indicative of Alternating Shaft Speed." Other U.S. Pat. Nos. 3,477,014 issued to A. L. Blythe on Nov. 4, 1969, and entitled "Electrical Control Systems With Stabilizing Control Means;" 2,981,882 issued Apr. 25, 1961, to M. Rosenblatt entitled "Stabilizing circuit for Dynamoelectric Machines;" and 3,656,048 issued Apr. 11, 1972 to A. W. Hauf entitled "Non-Linear Exciter Controller for Power System Damping" propose methods for stabilizing low frequency power oscillation by measuring some parameter of a system and operating on the field excitation of the generator for corrective action. It would be advantageous if a method and apparatus could be found which would sense torsional oscillation and quickly operate on the output power of the synchronous generator to thus providing compensation.